Julia Llewellyn Smith

RECENTLY, at a party, I found myself talking to "Alice", a woman who seemed very like me. In her early 40s, married, two children under 10 – work she fitted around them. Alice seemed distracted, I was tired, conversation flagged. Soon afterwards, I left.

The hostess told me what happened next. "Alice made a beeline for X [an eligible bachelor], and listened enraptured to him talking about his book on a subject she couldn't care less about. The next morning she called, demanding his number. She said she was going to have an affair with him."

I hadn't felt so behind the times since year 10 when friends started wearing Converse high-tops, while I still favoured Clarks' Mary Janes. While I was discussing Downton Abbey, Alice had been selecting a lover. I'd been wondering if I'd catch the Tube or a night bus, she'd been mentally donning her Agent Provocateur thong.

Modern her. Fuddy me. Because infidelity, it seems, is "hot" for 2013, like stripes and bermuda shorts. We chastise celebrity "cheaters", such as Tiger Woods, but research reveals that about 34 per cent of women have had an affair, and 32 per cent of men. A host of scholarly tomes have just been published rejecting monogamy.

Catherine Hakim's The New Rules: Internet Dating, Playfairs and Erotic Power argues that, far from being the province of hippies, open marriages or "playfairs" are the chic, French way to endure the monotony of married life.

Less dogmatically, in Rewriting the Rules psychologist Dr Meg Barker encourages us to rethink the norms underlying our love lives, from rigid definitions of sexuality to the idea that there is only one person for us out there.

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If open marriages appeal to you both, yippee. After all, divorce rates are high. We live longer, meaning a potential 60-plus years yoked to one person, as opposed to five years for a mediaeval peasant.

Our expectations of marriage have altered dramatically, too. Just a century ago, we married to seal alliances and pass on wealth; now, we live in a secular society where women don't need providers. Yet while we've divested marriage of much of its practical purpose, we've overinvested it with romantic hopes. Our future spouse must be a soul mate, if not, he can move on.

Last year, a 41-year-old female banker divorced her husband, not because he was unkind, but because he "forgot Valentine's Day". Inspired by the erotic bestseller Fifty Shades of Grey, she bought some sexy underwear, but he didn't notice, which was grounds for "unreasonable behaviour".

The issue of gay marriage has, of course, prompted vociferous debate about what this most fundamental of institutions is, and what it isn't. But to a greater or lesser degree, marriage in some aspects has been quietly, even unconsciously, renegotiated by many of us in the West.

A modern marriage, with its notion of equality of the sexes, and of being all things to each other, is a different beast from the marriages of our grandparents or great-grandparents. Did their clear gender hierarchy (antipathetic as it may be to 21st-century minds), plus the reciprocal practical requirements of men and, above all, women, make our forebears more clear-sighted about married life? Are we asking too much of it? Do we need to rethink?

"In the recent past there's been a huge growth of this idea that you need one perfect relationship; that you will get everything from it – romance and children, financial stability and friendship and a great sex life," says Barker.

"No other generation had such huge hopes invested in just one relationship and it is an enormous ask." In other words, the scaffolding that supports our society is now carrying so much baggage it threatens to collapse.

Popular culture constantly reminds us that there's a huge gap between the "you complete me" moment when a couple finally kiss, and actual married life – depicted in films as a desert of nagging, sexual unfulfilment and filthy nappies. But it's not just a cultural thing.

Biochemistry also ensures falling in love is far more fun than a long-term relationship. In the early throes of passion your brain is flooded with dopamine, making you behave like someone on crack cocaine. When the veil of oxytocin wears off after a year or so, however, you begin to see your partner as they really are and vice versa. This is when we need to make choices.

To Hakim, it's a stark division between dumping your spouse or embarking on a discreet affair to "reconnect with your happy single self". Technology has made it easier than ever, with plenty of "affairs" websites that allow married people to plot liaisons. Over the years, I've met users of such sites who don't want the financial and emotional upheaval of divorce, but who – as Alice (who was rejected by the bachelor, but had a fling with a school-gate father) told my friend – need to answer the nagging question "Is this all there is?"

Sex therapist Paula Hall agrees that the "old" marriage model may be becoming unworkable: "Affairs usually cause pain, because one person feels betrayed. Yet experiences shift with age, the things we wanted in our early 20s are not the same things we want in our 40s, so maybe we need an alternative approach; maybe marriage shouldn't be for life."

But what about the children? "If our social norms changed that might be less of an issue. The worst thing about divorce for children is the conflict usually involved, which means a lack of ongoing contact with one parent," Hall says.

"If we could find a way to negotiate divorce without conflict, there might not be a problem."

She adds: "I'm not promoting this. But if a relationship has died, you have to look at the best way of handling that death."

If even Hall sounds so gung-ho, when her job, I'd assumed, was to persuade jaded couples to plod on, perhaps I should follow Alice's example. My husband, James, and I have been a couple for 13 years, married for six. Our register-office wedding was so low-key that several friends learnt about it years later. I was eight months pregnant. Our oldest daughter was two and sat on my lap confiding loudly to the tiny audience: "I got diarrhoea."

I remember our anniversary because it's April 5, the end of the tax year. Apt, because we got "engaged" (there was no proposal or ring) after a conversation about how if one of us died, the grieving survivor – owing to our cohabiting status – would end up paying inheritance tax on our house.

Yet I wonder if our unromantic nuptials are the reason we're still together – and (touch wood) happy. I hated the idea of a big wedding, put off after witnessing expensive shindigs where X Factor runners-up sang and fireworks spelt out the happy couple's name in the sky.

My unease was that by buying into the wedding industry's "one perfect day" hype, these brides were also subscribing to a dangerous myth of "the one", the myth Barker warns against and that I'd rejected.

I was 36 when I married, ancient by my mother's standards (she was 22), but normal in my middle-class circle. Like all my friends, I'd been in a handful of fairly long-term relationships. They failed for various reasons, but mainly because when the early gloss faded, I'd moved on.

There was a huge spark when I met James, but by then I was also mature enough to know, like Hardy's Bathsheba Everdene, that enduring love isn't about the flashy Sergeant Troys but the solid Gabriel Oaks, with romance growing "in the interstices of a mass of hard, prosaic reality".

Yet popular belief is that marriage will bring permanent bliss. Equally, if things are imperfect, we're persuaded that divorce will make us happy. Obviously, no one should stay married to a drunk or a wife-beater. But the truth – as Granny told us – is that happiness virtually never comes from others, but from within. Research shows that even divorced people who say they were unhappily married aren't necessarily happier five years later.

Arranged marriages are about as unromantic as a tax return, but they have very low divorce rates, partly because of cultural expectations, but also, as Brian J. Willoughby of the School of Family Life at Brigham Young University explains, because they "start cold and heat up and boil over time as the couple grows. Non-arranged marriages are expected to start out boiling hot but many eventually find that this heat dissipates and we're left with a relationship that's cold."

I'm glad I waited to marry, but the pressure's mounting for women to avoid my selfish example, to settle young or risk withered ovaries. Yet those "tick-tock fertility timebomb" headlines are scaremongering – 89 per cent of women over 35 are fertile (and those who aren't may never have been). At 45, 13 per cent can still have children. And maths is on the side of older brides, with the risk of divorce dropping significantly in couples who marry over the age of 25.

In fact, although marriage rates are at their lowest since records began, divorce rates too have also declined to their lowest in many years. Since no one's pressured any more into marrying, fewer, it seems, want to escape its bonds. The glaring exception is among the baby-boomer over-60s.

"These are the people who invented free love, smoked pot and wore flowers in their hair," says David Pinless, the director of the dating agency Fiftyalready.com. "They simply aren't going to stay together for appearance's sake."

I can't be complacent: maybe James and I will emerge blinking from the trenches of raising children into a plateau where we have the time and energy to bleat: "What about me?"

But I dare to hope not. Having never wanted marriage, I'm now keen to go the distance – and not just because otherwise I'd have to learn to program the boiler.

I know at times it's going to be hard work, continually injecting romance into an institution that – ironically – needs to be de-romanticised if it's to survive. Yet I also believe that a happy-ever-after ending can, and should, be worked for.